Microwave dielectrics

ABSTRACT

A group of dielectric compounds and mixtures of dielectric compounds with useful properties at microwave frequencies is disclosed. The dielectric materials are temperature compensated, have relatively high dielectric constants, and comprise the group of BaTi4O9, and mixtures of Li2TiO3 and Li4Ti5O12, MgTi2O5 and TiO2, Zn2TiO4 and TiO2, and BaTi4O9 and TiO2.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 385,017, filed Aug. 2,1973 now abandoned, which is a continuation of application Ser. No.130,356 filed Apr. 1, 1971 (now abandoned).

BACKGROUND OF THE INVENTION

This invention relates to mixtures of dielectric compounds for use asmicrowave dielectrics at microwave frequencies such as XS and C band andhaving the properties of high dielectric constant, low dielectric lossat microwave frequencies and a low temperature coefficient of dielectricconstant. The above dielectric materials because of their dielectricproperties are highly desirable as substrates for microwave integratedcircuits where dielectric constant insensitivity to temperaturevariations is a requirement; for dielectric filters, for example, inwave guides; for dielectric loading of ferrite phase shifters at eitherX, S or C band operation and as a temperature compensated dielectric instripline applications such as stripline circulators in which a highdielectric constant at high frequency with low losses is a requirement.Additionally, such materials are useful as quarter wave plates at L banddue to the reduction in size because of the high relative dielectricconstant and even as a solid dielectric wave guide as a replacement foralumina which currently has wide application.

In the prior art such dielectric materials as BaTi₄ O₉ and zinc titanatehave been prepared, however, have not found use in the above-mentionedmicrowave applications because of difficulty encountered in fabricatingthe materials which results in wide variations in material dielectricproperties from batch to batch of finished material.

The present invention describes a method of manufacture of the abovehigh dielectric constant materials such that ease of fabricabilitycoupled with highly reproducible dielectric properties is obtainable atminimal cost and with the materials exhibiting high mechanical strengthand densities approaching theoretical density for the materials.

SUMMARY OF THE INVENTION

Methods of manufacturing BaTi₄ O₉, and mixtures of BaTi₄ O₉ and TiO₂,Li₂ TiO₃ and Li₄ Ti₅ O₁₂, MgTi₂ O₅ and TiO₂, and Zn₂ TiO₄ and TiO₂ aredisclosed in which the materials are prepared by mixing carbonatesand/or oxides of the constitutents in ball mills with appropriateallowance in the formulation to take into account the addition of one ofthe constitutents due to abrasion of the mill lining and media. Themixtures are calcined at temperatures of 800° to 1300° C. for a periodof up to 10 hours. Final firings are conducted at temperatures ofapproximately 1100° to 1450° C. for up to 10 hours in oxidizingatmospheres. As an alternative, hot pressing may be employed rather thanfiring or both firing and hot pressing may be utilized. Dielectricmaterials of high relative densities with low dielectric losses and arange of dielectric constant values and temperature coefficients of thedielectric constant from positive to negative and with near perfecttemperature compensation may be achieved. In particular, a novel methodfor the preparation of BaTi₄ O₉ is disclosed in which any desired shapeof the material may be produced such that the resultant dielectricproperties are almost identically reproducible from batch to batch, witheach manufactured piece of material having high mechanical strength andlow temperature coefficient. Two phase systems of mixtures ofdielectrics having positive and negative temperature coefficients whichproduce high dielectric constant compensated material are alsodisclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of the method of manufacturing BaTi₄ O₉ fromBaCO₃ and TiO₂ in accordance with the present invention;

FIG. 2 is a graph of the dielectric constant plotted against temperaturevariation for BaTi₄ O₉ ;

FIG. 3 is a graph of the X-band dielectric constant of BaTi₄ O₉ vs. thepercent theoretical density of the material;

FIG. 4 is a plot of the X-band temperature coefficient in ppm per degreecentigrade vs. the percent theoretical density for BaTi₄ O₉.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a flow chart of the method of manufacture ofBaTi₄ O₉ in accordance with the present invention is illustrated. Thebatch size is of course dependent upon the amount of BaTi₄ O₉ required;however, 250 gram and one kilogram batches have been prepared. The BaTi₄O₉ is prepared by mixing carbonates and/or oxides of the constitutentsin ball mills with appropriate allowance in the formulation to take intoaccount the additionof one of the constitutents due to abrasion of themill lining and media. The mixture is calcined at a temperature of900°-1150° for aperiod of up to 10 hours with final firing conducted attemperatures of 1200°-1400° for up to 10 hours in oxidizing atmospheres,andthe end product BaTi₄ O₉ is of high relative density, high dielectricconstant, low dielectric loss and has a temperature stable dielectricconstant.

Stoichiometric quantities of BaCO₃ and TiO₂ of reagent grade availablefrom the J. T. Baker Chemical Company are weighed and mixed in theproportion of one mole of BaCO₃ to four moles of TiO₂. Allowance must bemade for pick up from the mill to bring the fully processed batch to thedesired stoichiometery. The rate of pick up was, for example, typically0.09 grams of titanium per hour of milling, thus, approximately twograms of TiO₂ per one hundred grams of batch material are left out atthe weighing and mixing stage. The BaCO₃ andTiO₂ mixture is ball milledfor 16 hours in a titanium ball mill in aninert medium of methylalcohol, for example, at room temperature. The speedof the mill istypically 80 percent of the critical speed and the amount ofmethylalcohol is typically 60 percent by volume of liquid slurry, whichisbetween 130 to 150 percent of the weight of the mixture withoutalcohol. Ofcourse other inert substances such as water may be used toform the mechanical slurry.

After ball milling, the mills are discharged into open pans and themethyl alcohol removed by drying at about 60° C. The dried materials aregranulated through a 50 mesh stainless steel screen into an enamel panandform a cake like substance. Following drying and screening thegranulated mechanical mixture is calcined at approximately 1150° C. withthe calcine product appearing as a porous clinker or semisinteredsubstance. Following calcining the batches of BaTi₄ O₉ are again milledfor16 hours in a titanium ball mill in an inert methyl alcohol mediumunder conditions substantially identical to the first ball milling stepafter which the resulting BaTi₄ O₉ is again dried and granulated toinsure batch homogeneity. The BaTi₄ O₉ is then dry pressed in a mold atroom temperature at pressures between 1500 psi to 10,000 psi. Thematerial is then hot pressed to insure high density and/orconventionally fired at approximately 1200° C. by being brought up fromroom temperature at 100° C. per hour. The BaTi₄ O₉ is held at 1250° C.for 10 hours and then furnace cooled from between 18 to 24hours to roomtemperature. The firing was done in a pure oxygen atmosphere to insurethat titanium does not reduce. The final material shape may be eitherbar like, disc like or any other desired shape, such as cylindrical.

Prior to compacting, some of the powder from each batch may be placed inzirconia crucibles and heated several hours at 800° C. in an oxidizingatmosphere to convert to oxides any metallic titanium picked up duringthe second milling. Compacts of 0.625" diameter were formed in a steeldie at 8500 psi for hot pressed material with each compact being placedin a stabilized zirconia die measuring 0.75" diameter by 2" high.Thecompact was surrounded by stabilized zirconia such to eliminate stickingto the die walls or plunger faces, and the assembled die was placed in aresistance heated furnace with a pressure of 5700 psi applied to theplungers.

Referring now to FIG. 2, a plot of the dielectric constant variationwith temperature for BaTi₄ O₉ produced by the process described withrespect to FIG. 1 is illustrated. The temperature coefficient between-70° C. to +100° C. was constant giving a linear variation of dielectricconstant with temperature. The stability of the dielectric constant ofBaTi₄ O₉ is apparent from FIG. 3 with a dielectric constant value ofless than 38.1 at a temperature of -70° C. and a dielectric constantvalue of greater than 37.8 at +100° C.

Referring now to FIGS. 3 and 4, plots of the dielectric constant andtemperature coefficient of dielectric constant respectively as afunction of the percent theoretical density of BaTi₄ O₉ are plotted. Thedielectric constant extrapolates to l = 39.5 and the temperaturecoefficient to τ = -57 ppm per degree centigrade at theoretical density.It is apparent that for densities in excess of about 85 percenttheoretical, the temperature coefficient is essentially proportional tothe percent density. The dielectric constant was measured at X-band inboth FIGS. 3 and 4, however, substantially the same results wereobtained both at C-band (5.35 gigahertz) and S-band (2.68 gigahertz).BaTi₄ O₉ has an orthorhombic structure and an X-ray density of 4.54grams per cubic centimeter. Both hot pressed and conventionally firedmaterial as well as material from different batches when made inaccordance with the before described process consistently yield materialwith densities inexcess of 97 percent of theoretical. Dielectricconstant, dielectric loss and temperature coefficient are extremelyconstant, reproducible and predictable from batch to batch irrespectiveof whether conventional firing or hot pressing is used for densificationin fabrication. This consistency of properties makes BaTi₄ O₉ a highlyuseful high dielectric constant microwave material. The following tablecontains dielectric properties on batches of BaTi₄ O₉ made in accordancewith the instant method.

    __________________________________________________________________________    Sintering                                                                     or Hot Pressing                                                               Conditions    Density 1 MHz    X-Band                                         __________________________________________________________________________                                            τ                                 Batch                                                                             Temp Time Bulk                                                                              Per-     tan∫                                                                              tan∫                                                                         ppm                                   No. (° C.)                                                                      min  g/cc                                                                              cent                                                                              k    10.sup.-.sup.4                                                                    k    10.sup.-.sup.4                                                                    ° C.                           __________________________________________________________________________    1   1255 600  4.48                                                                              98.7                                                                              38.28                                                                              11.7                                                                              38.20                                                                              4.2 -50                                   2   1255 600  4.47                                                                              98.5                                                                              38.29                                                                              15.0                                                                              38.18                                                                              4.2 -55                                   3   1255 600  4.48                                                                              98.7                                                                              37.17                                                                              11.6                                                                              38.18                                                                              4.17                                                                              -54                                   4   1255 600  4.47                                                                              98.5                                                                              38.26                                                                              26.5                                                                              38.17                                                                              4.17                                                                              -55                                   5   1255 600  4.47                                                                              98.5                                                                              38.33                                                                               9.8                                                                              38.19                                                                              3.27                                                                              -56                                   6   1255 600  4.48                                                                              98.7                                                                              38.29                                                                               9.2                                                                              38.26                                                                              4.15                                                                              -55                                   7   1255 600  4.47                                                                              98.5                                                                              38.10                                                                               9.2                                                                              38.15                                                                              3.28                                                                              -50                                   8   1255 600  4.48                                                                              98.7                                                                              38.49                                                                              10.4                                                                              38.28                                                                              4.60                                                                              -50                                   9   1265 600  4.43                                                                              97.6                                                                              38.00                                                                              37.8                                                                              37.83                                                                              4.9 -54                                   10  1265 600  4.45                                                                              97.9                                                                              38.03                                                                              26.9                                                                              37.90                                                                              3.2 -49                                   __________________________________________________________________________

The averages for the above dielectric microwave data are l = 37.81±0.30, tanδ = 6.5± 3.0× 10⁻ ⁴ = 6.5± 3.0 and τ 53.4± 6.3 ppm per degreecentigrade. The consistency of properties from batch to batch is in partthe result of the reproducibility of the density and microstructure ofBaTi₄ O₉ inaccordance with the instant method. In a typical fired batchthe grain sizeis on the order of 10 microns or less and although thegrains are not equi-axed, which is normal for a non-cubic, material thegrain size is small enough that intergranular cracking caused byanisotropic thermal expansion of the grains is not observed. Thisproduces a body with good mechanical properties. The absence ofintergranular cracking also leads toa minimum number of grain pull outsduring any subsequent polishing which coupled with a noticeable lack ofporosity makes the material very amenable to polishing for microwaveintergrated circuit substrate applications. The following table is alist of modulus of rupture measurements on BaTi₄ O₉ samples made on anInstron tensile tester model TT-CM-l in four point bending with thesamples 2" long with the 2 loading points at 1" separations. At a crosshead speed of 0.05 centimeters per minute the modulus of rupture offracture strength, σ_(f) = 1.5p/BH2 where p is the load at fracture, Bthe sample width and H the sample thickness.

    ______________________________________                                        Sample  Width     Thickness Load                                              No.     b in.     h in.     p lb.   σ.sub.f psi                         ______________________________________                                        1       0.3039    0.1448    66.5    15,654                                    2       0.3040    0.1266    43.0    13,301                                    3       0.3041    0.1282    53.0    15,906                                    4       0.3072    0.1281    50.0    14,878                                    5       0.3070    0.1281    57.0    16,985                                    6       0.3067    0.1246    46.4    14,613                                    7       0.3068    0.1273    62.4    18,875                                    8       0.3042    0.1247    52.4    16,617                                    9       0.3033    0.1248    54.4    17,273                                    10      0.3038    0.1362    53.3    15,524                                    11      0.3060    0.1286    52.2    15,471                                    ______________________________________                                    

The average value of σ_(f) is 15,918 ± 1423 psi.

The average thermal expansion coefficient of BaTi₄ O₉ from roomtemperature to 900° C. has been found to be α = 9.4× 10⁻ ⁶ /°C.-¹. Thethermal expansion coefficient Δl/l 0×10⁻ ³ ranged linearly fromapproximately 1.2 at 200° C. to 9.0 at 1000° C. The following table is asummary of the properties of BaTi₄ O₉ made in accordance with thepresent invention.

    ______________________________________                                        Composition (single phase)                                                                            BaTi.sub.4 O.sub.9                                    Water Absorption (percent)                                                                            <0.00                                                 Specific Gravity (percent theoretical)                                                                98.00 ± 0.57                                       Color                   Tan                                                   Hardness (Moh's)        7                                                     Coefficient of Thermal Expansion (° C..sup.-1)                                                 9.4 × 10.sup.-.sup.6                            Modulus of Rupture (psi)                                                                              16,000 ± 1400                                      Dielectric Constant (X-Band)                                                                          37.81 ± 0.30                                       Dielectric Loss Tangent (X-Band)                                                                      6.5 ± 3.0 × 10.sup.-.sup.4                   Temperature Coefficient of Dielectric                                         Constant (X-Band, ppm ° C.sup.-1)                                                              -53.4 ± 6.3                                        Temperature Coefficient                                                       (corrected for sample expansion)                                                                      -60.31 ± 6.3                                       ______________________________________                                    

Another class of temperature compensated high dielectric constantmaterial is obtained from the Li₂ O-TiO₂ system. This material is a twophase composition of 48 volume percent Li₂ TiO₃ and 52 volume percentLi₄ Ti₅ O₁₂, having a dielectric constant of approximately 26, and whichmay be produced by substantially the same method as that previouslydescribed for producing BaTi₄ O₉. The dielectric properties and densityare both unaffected by processing conditions and batch to batchreproducibility with consistency of properties may be obtained. Thefollowing is a table of the microwave dielectric properties of thelithium titanate dielectric material obtainedon 9 batches producedeither by sintering or hot pressing:

    __________________________________________________________________________    Sintering                                                                     or Hot Pressing                                                               Conditions    Density 1 MHz    X-Band                                         __________________________________________________________________________                                            τ                                 Batch                                                                             Temp Time Bulk                                                                              Per-     tan∫                                                                              tan∫                                                                         ppm                                   No. (° C.)                                                                      min  g/cc                                                                              cent                                                                              k    10.sup.-.sup.4                                                                    k    10.sup.-.sup.4                                                                    ° C.                           __________________________________________________________________________    1   1270 600  3.34                                                                              96.2                                                                              26.79                                                                              35.3                                                                              25.82                                                                              2.33                                                                              -38                                   2   1270 600  3.36                                                                              96.9                                                                              26.68                                                                              17.3                                                                              25.79                                                                              1.29                                                                              -41                                   3   1260 600  3.34                                                                              96.1                                                                              26.67                                                                              21.3                                                                              26.32                                                                              3.3 -43                                   4   1260 600  3.28                                                                              94.5                                                                              26.04                                                                              40.5                                                                              25.79                                                                              1.39                                                                              -35                                   5   1260 600  3.28                                                                              94.5                                                                              25.89                                                                              41.2                                                                              25.77                                                                              2.4 -34                                   6   1255 600  3.30                                                                              95.0                                                                              25.87                                                                              44.2                                                                              25.95                                                                              1.3 -41                                   7   1255 600  3.30                                                                              95.0                                                                              25.38                                                                              47.6                                                                              25.95                                                                              5.5 -37                                   8   1255 600  3.31                                                                              95.4                                                                              25.66                                                                              49.7                                                                              25.99                                                                              2.4 -38                                   9   1255 600  3.32                                                                              95.6                                                                              26.21                                                                              49.5                                                                              26.22                                                                              4.3 -41                                   __________________________________________________________________________

As is apparent from the above table, the microwave losses of thismaterial are exceedingly low and consistent. The modulus of rupture oflithium titanate has been found to be 6664 ± 3109 psi when measured onan Instron tensile tester in the same manner as was the BaTi₄ O₉material. The thermal expansion coefficient of the lithium titanate wasfound to be 19.37× 10⁻ ⁶ °C.⁻¹ when measured between 200° and 900° C.The following is a summary of properties of the two phase composition ofLi₂ TiO₃ and Li₄Ti₅ O₁₂ :

    ______________________________________                                        Composition (two phase)                                                                              Li.sub.2 O . 1.5 TiO.sub.2                             Water Absorption (percent)                                                                           0.12 ± 0.08                                         Specific Gravity (percent theoretical)                                                               94.6 ± 1.9                                          Color                  White                                                  Hardness (Moh's)       <6                                                     Coefficient of Thermal Expansion                                                                     19.4×10.sup.-.sup.6                              (° C..sup.-1)                                                          Modulus of Rupture (psi)                                                                             6700 ± 3100                                         Dielectric Constant (X-Band)                                                                         25.65 ± 0.88                                        Dielectric Loss Tangent (X-Band)                                                                     2.55 ± 1.76×10.sup.-.sup.4                    Temperature Coefficient of Dielectric                                         Constant (X-Band, ppm ° C..sup.-1)                                                            -39.6 ± 4.2                                         Temperature Coefficient                                                       (corrected for sample expansion)                                                                     -53.69 ± 4.2                                        ______________________________________                                    

Another class of temperature compensated microwave dielectric materialhas been found to be a mixture of MgTi₂ O₅ and TiO₂, which material mayeffectively be prepared by the same method as is BaTi₄ O₉, that is bymixing oxides of the constituents in ball mills with appropriateallowance in the formulation to take into account the additionof one ofthe constituents due to abrasion of the mill lining and media. Themixture is then calcined at temperatures of approximately 1400°for 10hours in an oxidizing atmosphere and then either hot pressed orconveniently fired for densification with drying and screening after theball milling step. This material offers a relatively high dielectricconstant with low dielectric loss and high density. Stoichiometricquantities of magnesium titanate with 16 volume percent free TiO₂arerequired. The following table lists the dielectric properties ofMgTi₂O₅ -TiO₂ as measured on 9 batches made in accordance with themethod of the instant invention.

    __________________________________________________________________________    Sintering                                                                     or Hot Pressing                                                               Conditions    Density 1 MHz      X-Band                                       __________________________________________________________________________                                               τ                              Batch                                                                             Temp Time Bulk                                                                              Per-     tan∫  tan∫                                                                          ppm                                No. (° C)                                                                       min  g/cc                                                                              cent                                                                              k    10.sup.-.sup.4                                                                      k    10.sup.-.sup.4                                                                     ° C.                        __________________________________________________________________________    1   1410 600  3.53                                                                              94.9                                                                              22.10                                                                              21.1  21.30                                                                              2.93 +45                                2   1410 600  3.56                                                                              95.4                                                                              22.77                                                                              25.3  22.13                                                                              2.13 +46                                3   1410 600  3.58                                                                              95.7                                                                              24.50                                                                              107.7 23.13                                                                              1.25 +12                                4   1410 600  3.61                                                                              96.3                                                                              24.68                                                                              22.3  24.29                                                                              5.25 + 1.3                              5   1410 600  3.62                                                                              96.0                                                                              25.98                                                                              20.3  25.40                                                                              4.38 -22                                6   1390 600  3.62                                                                              96.5                                                                              26.76                                                                              242.  24.17                                                                              14.4 + 2.2                              7   1390 600  3.62                                                                              96.5                                                                              25.36                                                                              139.  24.15                                                                              10.3 + 5.5                              8   1410 600  3.56                                                                              94.9                                                                              24.84                                                                              242.  23.60                                                                              4.2  + 9                                9   1410 600  3.56                                                                              94.9                                                                              24.18                                                                              63.2  23.62                                                                              4.19 +11                                __________________________________________________________________________

A plot of the dielectric constant and temperature coefficient ofmagnesium titanate vs. the volume fraction of TiO₂ indicates thatperfect compensation should occur at approximately 16.5 volume percentTiO₂.

A summary of the dielectric properties of magnesium titanate appears inthetable below:

    ______________________________________                                        Composition (two phase)                                                                              84 v/o MgTi.sub.2 O.sub.5 -                                                   16 v/o TiO.sub.2                                       Water Absorption (percent)                                                                           0.43 ± 0.12                                         Specific Gravity (percent theoretical)                                                               95.9 ± 0.80                                         Color                  Gray                                                   Hardness (Moh's)       <7                                                     Coefficient of Thermal Expansion                                                                     1.87 × 10.sup.-.sup.6                            (° C..sup.-1)                                                          Modulus of Rupture (psi)                                                                             2723 ± 2.4                                          Dielectric Constant (X-Band)                                                                         23.97 ± 0.33                                        Dielectric Loss Tangent (X-Band)                                                                     7.7 ± 4.5 × 10.sup.-.sup.1                    Temperature Coefficient of Dielectric                                         Constant (X-Band, ppm ° C..sup.-1)                                                            +5.8 ± 4.2                                          Temperature Coefficient                                                       (corrected for sample expansion)                                                                     +4.4 ± 4.2                                          ______________________________________                                    

An additional dielectric material which may be fabricated by the processdescribed with respect to the method of manufacturing BaTi₄ O₉ shown inFIG. 1 is a two phase mixture of Zn₂ TiO₄ and TiO₂. Zn₂ TiO₄ has apositive temperature coefficient and TiO₂ a negative temperaturecoefficient, therefore the two phase mixture of the two compositionsresults in a compensated material. The following table lists thedielectric properties of a Zn₂ TiO₄ -TiO₂ system as measured on a samplemode in accordance with the instant invention:

    __________________________________________________________________________    Sintering                                                                     or Hot Pressing                                                               Conditions    Density                                                                              1 MHz    X-Band                                          __________________________________________________________________________                                            τ                                 Batch                                                                             Temp Time Bulk                                                                              Per-    tan∫                                                                              tan∫                                                                          ppm                                   No. (° C.)                                                                      min  g/cc                                                                              cent                                                                             k    10.sup.-.sup.4                                                                    k    10.sup.-.sup.4                                                                     ° C.                           __________________________________________________________________________    1   1280 600  4.84   19.69                                                                              44.6                                                                              19.05                                                                              8.32 +104                                  2   1280 600  4.83   19.71                                                                              45.7                                                                              19.13                                                                              9.24 +104                                  3   1280 600  4.79   19.75                                                                              43.4                                                                              19.22                                                                              11.0 +103                                  4   1250 600  4.88   19.86                                                                              43.4                                                                              19.01                                                                              10.0 + 88                                  5   1250 600  4.86   20.02                                                                              38.3                                                                              19.24                                                                              10.3 +109                                  6   1250 600  4.94   24.55                                                                              72.3                                                                              23.92                                                                              9.3  - 8                                   7   1260 600  4.89   24.71                                                                              36.3                                                                              24.03                                                                              7.2  - -                                   8   1265 600  4.89   24.58                                                                              36.8                                                                              24.00                                                                              8.3  - 12                                  9   1265 600  4.89   24.58                                                                              38.4                                                                              23.96                                                                              8.2  - 17                                  __________________________________________________________________________

For a compensated system the weight fraction of Zn₂ TiO₄ should beapproximately 75 percent with 25 percent TiO₂. A summary of themicrowave dielectric properties of the zinc titanate system is listedbelow, in which the fraction of Zn₂ TiO₄ is 75 percent.

    ______________________________________                                        Composition (two phase)                                                                              45.19 mole percent                                                             TiO.sub.2                                             Water Absorption (percent)                                                                           <0.00                                                  Specific Gravity (g/cc)                                                                              4.900 ± 0.023                                       Color                  Light Yellow                                           Hardness (Moh's)       ˜7                                               Coefficient of Thermal Expansion                                                                     11.47 × 10.sup.-.sup.6                           (° C..sup.-1)                                                          Modulus of Rupture (psi)                                                                             13,294 ± 1711                                       Dielectric Constant (X-Band)                                                                         23.97 ± 0.04                                        Dielectric Loss Tangent (X-Band)                                                                     8.25 ± 0.89×10 .sup.-.sup.4                   Temperature Coefficient of Dielectric                                                                -17.0 ± 10.0                                        Constant (X-Band, ppm ° C..sup.-1)                                     Temperature Coefficient                                                                              -25.3 ± 10.0                                        (corrected for sample expansion)                                              ______________________________________                                    

The materials described above are of high relative densities with lowdielectric losses and a range of dielectric constant values andtemperature coefficients of the dielectric constant from stronglypositiveto strongly negative including perfect temperature compensation.The dielectric constants at X-band range from 16 to 100, the temperaturecoefficient of dielectric constant ranges from +200 to -800, and thedielectric loss tangent at X-band is less than 9× 10⁻ ⁴. Composite highdielectric composite materials are disclosed in which the dielectriccharacteristics of two phase compositions are the result ofthecombination of properties of the constituent materials.

While particular embodiments of the invention have been shown anddescribed, various modifications thereof will be apparent to thoseskilledin the art. For example, the dielectric constant of BaTi₄ O₉ maybe tailored to fall within the range of approximately thirty-eight toone hundred by mixing in varying amounts of TiO₂ thereby forming acomposite material with a more negative temperature coefficient.Therefore, it is not intended that the invention be limited to thedisclosed embodiments of details thereof and departures may be madetherefrom within the spirit and scope of the invention as defined in theappended claims.

What is claimed is:
 1. A method of forming a dielectric from titaniumdioxide comprising the steps of:mixing stoichiometric quantities of TiO₂and a carbonate drawn from the class of carbonates consisting of BaCO₃,Li₂ CO₃, MgCO₃ and ZnCO₃ to provide a mixture thereof; milling saidmixture in a ball mill containing a liquid medium to produce a liquidslurry; drying said liquid slurry to produce a dried mixture;granulating said dried mixture; calcining said granulated mixture at atemperature within the temperature range of 800° C. to 1300° C. toprovide a chemical reaction between said stoichiometric quantities ofTiO₂ and of the carbonate of said class of carbonates wherein there isformed a product drawn from the class of substances consisting of BaTi₄O₉, a mixture of Li₂ TiO₃ and Li₄ Ti₅ O₁₂, MgTi₂ O₅ --TiO₂ and Zn₂ TiO₄--TiO₂ ; ball milling said product to produce a powder thereof;compressing said powdered product in a mold with pressure in the rangeof 1500 to 10,000 pounds per square inch to increase the density anddielectric constant of said powder; and firing said compressed powder inan oxidizing atmosphere at a temperature in the temperature range of1100° C. to 1450° C. until the density of said compressed powder hasexceeded approximately 85% of the theoretical maximum density, therebystabilizing the dielectric constant of said compressed powder withrespect to variations in the temperature of said compressed powder.